The present invention relates to a method of manufacturing an optical crystal element used in a semiconductor laser (LD) excitation solid-state laser device, and more specifically, relates to a method of manufacturing an optical crystal element having functions as a laser medium, resonator, and etalon, in a microchip laser.
In recent years, a short wavelength laser such as a green laser and blue laser has been focused in a broad range of fields including an interferometer, an optical disc pick-up, and a printer device. Accordingly, research and development for laser devices generating such laser light has been actively pursued. In a known short wavelength laser device, a wavelength conversion element is inserted in a path of fundamental wave laser light for generating high-order harmonic wave laser light to be emitted outwardly. FIG. 4 is a schematic view showing a structure of the laser device.
The laser device comprises a semiconductor laser (LD) 1 for generating excitation light; a lens 2 for focusing excitation light from the semiconductor laser 1; a laser medium 3 to be excited by excitation light for inducing and emitting laser light including fundamental wave light; a wavelength conversion element 4 formed of a non-linear optical crystal or the like for generating high-order harmonic light (especially second harmonic generation; SHG) from fundamental wave light from the laser medium 3; an etalon 5 for selectively transmitting light with a specific wavelength; and an output mirror 6 for reflecting light and transmitting a part of light.
Excitation light from the semiconductor laser 1 is conversed through the lens 2, and is irradiated on the laser medium 3. In the laser medium 3, a reflective layer 3a is formed on an incident plane of excitation light for efficiently transmitting excitation light and reflecting fundamental wave and high-order harmonic wave light at a high reflectance. The reflective layer 3a and the output mirror 6 comprise an optical resonator. The laser medium 3 induces and emits fundamental wave light through excitation light, and fundamental wave light oscillates and is amplified in the optical resonator. The laser medium 3 is primarily formed of, for example, Nd:YAG (hereinafter simply referred to as YAG) for oscillating blue light with a wavelength of 946 nm, or Nd:YVO4 (hereinafter simply referred to as YVO4) for oscillating green light with a wavelength of 1064 nm.
When fundamental wave light passes through the wavelength conversion element 4, high-order harmonic wave light is generated. Accordingly, fundamental wave light and high-order harmonic wave light coexist within the optical resonator. The output mirror 6 reflects fundamental wave light, while high-order harmonic wave light passes through the output mirror 6. Accordingly, as shown in the figure, only high-order harmonic wave laser light is irradiated to a right side of the output mirror 6 as a laser light output. During the oscillation process, high-order longitudinal oscillation modes (i.e., numerous oscillation wavelengths) are generated in laser light. The etalon 5 having a high transmittance is inserted into the optical resonator. Accordingly, among multiple oscillation spectra, it is possible to obtain only single longitudinal mode laser light with a desired wavelength and high coherent in a space region and a frequency (or wavelength) region.
In the laser device, it is necessary to accurately arrange the laser medium 3, the resonator (output mirror 6), the wavelength conversion element 4, and the etalon 5 at predetermined locations. Accordingly, it is difficult to reduce an overall size of the device. Further, it is necessary to provide an adjustment mechanism for adjusting a relative arrangement of the optical elements and a temperature controlling device for preventing temperature fluctuation due to an ambient temperature, thereby further increasing a size. Also, the device tends to have a complex configuration, thereby increasing cost.
In view of the problems, a laser device known as a microchip laser has been developed recently (see Non-Patent Reference 1). The microchip laser is provided with an extremely thin laser medium (usually about 1 mm or less in thickness), so that the laser medium has functions of a resonator and an etalon. FIG. 5 is a schematic view of a microchip laser device. The semiconductor laser 1 radiates excitation light, and excitation light is conversed through the lens 2 and irradiated on an optical crystal element 7 of the microchip laser. An entrance side reflective layer 7a and an exit side reflective layer 7b are formed on surfaces of the optical crystal element 7, respectively. When fundamental wave light is reflected between the two reflective layers 7a and 7b, an oscillation frequency is unified, so that laser light of a specific wavelength is emitted to the right side. With such a structure, it is not necessary to dispose the resonator and the etalon externally, thereby reducing a size of the laser device and eliminating cumbersome position adjustment.
As described above, the optical crystal element 7 of the microchip laser has a thickness smaller than that the laser medium 3 of the regular LD excitation solid-state laser device. It is necessary to control the thickness with high precision to obtain a desired property. Conventionally, an optical crystal formed of YAG or the like is formed to have a desired thickness with a mechanical polishing process to produce the laser medium. In the conventional method, it is difficult to achieve thickness accuracy for an optical crystal of a microchip laser. As another processing method, an ion light etching using argon (Ar) is generally known. However, an etching rate for an optical crystal formed of YAG or the like is extremely low, and the method is impractical in terms of productivity.
Patent Reference 1 has disclosed an etching method with efficient and good control in surface roughness. In the method, ion beam etching is conducted in a mixture of CHF3 and Ar to control a thickness of an etalon formed of SiO2. The optical crystal formed of YAG or the like has a physical property different from that of a glass formed of SiO2. Accordingly, it is difficult to apply the method disclosed in Patent Reference 1 without adjusting an etching condition.
Patent Reference 1: Japanese Patent Publication (Kokai) No. 2003-304019
Non-Patent Reference 1: “Research and Development of Microchip Solid-state Laser”, Shinji Motokoshi, [Online] Laser Cross, June 2001, No. 160, Institute of Laser Technology, searched on Jan. 21, 2004, Internet <http://www.ilt.or.jp/cross/no160.pdf>
In view of the problems described above, an object of the present invention is to provide a method of manufacturing an optical crystal element of a laser device, in which it is possible to control a thickness of the crystal element with high precision while maintaining high productivity.
Further objects and advantages of the invention will be apparent from the following description of the invention.